In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity.
The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
In conjunction with Long Term Evolution (LTE) the use of relay base station nodes is described, e.g., in 3GPP LTE Rel-10. As shown in FIG. 1, a relay base station node (RN) communicates over the radio or air interface (e.g., the Uu interface) with one or more wireless terminals, and over an interface known as the Un interface with a donor base station node, e.g., a donor dNodeB. Thus, transmissions between UE and relay are done over the radio interface denoted Uu, which is the same as for regular eNB to UE communication. Such being the case, from a UE perspective a relay appears a regular eNB. Between the relay and the eNB, transmissions are performed over a radio interface denoted Un, which reuses much of the functionality of the Uu interface. This means that the DeNB handles the relay as a UE using the same protocols as when communicating with a UE, with some additions.
In general, a relay node (RN)) has the following characteristics:                The relay node (RN) controls cells, each of which appears to a UE as a separate cell distinct from the donor cell        The cells have their own Physical Cell ID (defined in LTE Rel-8) and the relay node shall transmit its own synchronization channels, reference symbols, . . . .        The UE receives scheduling information and HARQ feedback directly from the relay node and send its control channels (SR/CQI/ACK) to the relay node        The presence and function of the relay node (RN) does not impact UEs. Moreover, all legacy LTE UEs can be served by the relay cell.        
If the transmissions on the Un interface and the Uu interface (e.g., in the relay cell) are performed within the same frequency band, the relays are referred to as “inband relays”. In case the transmissions are on a separate frequency band, the relays are referred to as outband relays.
Transmissions over the Un interface and the Uu interface typically occur in frames. Each frame usually comprises plural subframes. A subframe may comprise a signaling portion and a data portion, with the data portion often being used to include or transmit, among other things, one or more data transport blocks, or simply “transport blocks” (TBs). In general, once a HARQ process has acknowledged successful reception of a transport block, the next transport block is prepared by the sending node and is then subject to HARQ processing.
To enable inband relays to be functional, the relay base station node should not transmit and receive at the same time on the same frequency as the donor base station node, since this could cause severe (self) interference. To preclude such same-time transmission and attendant interference, transmissions over the backhaul link (over the Un interface) and the access links (over the Uu interface) are time multiplexed in a manner intended to avoid interference. In this regard, the donor base station node configures, via a Radio Resource Control (RRC) procedure called “RN reconfiguration” (relay node reconfiguration), a so-called “RN subframe configuration” (relay node subframe configuration) in the relay base station node, which governs, among other things, which subframes are used for the backhaul link.
The subframes that can be used for backhaul communication (e.g., across the Un interface) are referred to as the “subframe configuration pattern”. The subframe configuration pattern may be communicated or signaled by the Radio Resource Control (RRC) protocol. For example, a change of a subframeConfigPattern information element or parameter in a message or signal generated by the RRC protocol may be used to communicate the subframe configuration pattern. The subframeConfigPattern parameter is defined, e.g., in 3GPP TS 36.331, section 6.3.2., as “RN-SubframeConfig”, and is a parameter which may differ for FDD (an 8-bit bitmap) and TDD (an integer).
For Frequency Division Duplex (FDD), the subframe configuration pattern is a bitmap, which together with the Multi-Media Broadcast over a Single Frequency Network (MBSFN) configurability in the RN cell, specified which downlink (DL) subframes that are configured for backhaul communication. Uplink (UL) subframes for backhaul communication are derived from the DL subframes for backhaul communication such that there is an UL backhaul subframe n if subframe n−4 were a DL backhaul subframe.
For Time Division Duplex (TDD), the subframe configuration pattern is an index referring to an explicitly specified subframe pattern in the 3GPP specification 3GPP TS 36.216, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer for relaying operation,” v.10.1.0 (the “36.216 Specification”), which is incorporated herein by reference in its entirety, specifying both DL and UL backhaul subframes.
For the downlink, to enable the relay to not transmit anything in its own cell (on the Uu interface) during the subframes used for the backhaul link (Un), the relay cell configures these subframes as MBSFN subframes. During an MBSFN subframe, the UEs in the relay cell do not expect to receive any cell-specific reference signals from the relay beyond what is transmitted in the first one or two Orthogonal Frequency Division Multiplexing (OFDM) symbols of the subframe. The relay node not scheduling any data in these subframes enables the relay node to listen to the downlink transmissions on the Un interface during the rest of these subframes (which are hence used for carrying downlink data from the donor base station node to the relay base station nodes). Conversely, subframes not configured for backhaul communication are used in the relay cell, and not used for the backhaul link. The donor base station node knows that these subframes are not available for backhaul communication and hence does not schedule the relay base station node
An example of time division between the Un and Uu interfaces is illustrated in FIG. 2. For example, FIG. 2 shows a series of blocks representing subframes, with shaded ones of the subframes being MBSFN subframes. FIG. 2 thus depicts the time multiplexed downlink (DL) transmissions on the Un interface from the donor base station node to the relay base station node (such transmissions being represented by the arrows above the blocks in FIG. 2) and the transmissions on the Uu interface from the relay base station node to a wireless terminal (such transmissions being represented by the arrows below the blocks of FIG. 2).
In Long Term Evolution (LTE), all regular data transmissions between a base station node and a wireless terminal, whether between a donor base station node (e.g., a dNB) and a wireless terminal or between a relay base station node (RN) and a wireless terminal, are protected by a process known as Hybrid Automatic Repeat request (HARQ). In the wireless terminal and the eNB/RN, respectively, there are a number of HARQ processes available for the downlink (DL), and a number of HARQ processes available for the uplink (UL).
Each HARQ process typically is assigned or associated with a HARQ process number. The downlink (DL) HARQ process number is part of the downlink (DL) assignment so it does not have to depend on the subframe number, e.g., is not necessarily correlated with and can even be independent of the subframe number. The uplink (UL) HARQ process number is mapped to subframes according to the 3GPP TS 36.216 standard (section 7.3).
Moreover, each HARQ process has a HARQ process “state”. As used herein, “HARQ process state” refers to contents of a buffer and value of at least one state variable or parameter, including a variable or parameter known as the New Data Indicator (NDI). The NDI parameter is a 1-bit value that is maintained for each HARQ process. In terms of a downlink (DL) HARQ process in the relay base station node, the HARQ process state comprises or is associated with a soft buffer which includes current soft-decoded information and the NDI. For an uplink (UL) HARQ process, the HARQ process state in the relay base station node comprises or is associated with a buffer for a MAC PDU to be transmitted and state variables, including the NDI and optionally including other variables such as the number of times a MAC PDU has been transmitted, current redundancy version and HARQ feedback. The downlink (DL) HARQ process in the donor base station node is the sending process, and hence is similar to the UL HARQ process in the relay node. In like manner, the uplink (UL) HARQ process in the donor base station node is similar to the downlink (DL) HARQ process in the relay base station node.
In the DL, the HARQ process number and the New Data Indicator (NDI) are signaled explicitly as two separate variables. The network signals if a transmission is a new transmission or a retransmission through or by the NDI. The NDI (being a 1-bit indicator) is/may be considered to be toggled (new transmission) or not toggled (retransmission) compared to its previous value.
In the uplink (UL), the HARQ process number is instead derived from the subframe in which it is, and the same HARQ process is tied to subframe n, n+8, n+16, etc. The concept of an NDI is also used in the uplink (UL) in essentially the same way as in the downlink (DL).
In view of the foregoing it will be understood that, for a relay base station node with an RN subframe configuration, only a subset of the subframes is available for transmissions on the backhaul link. This means fewer HARQ processes are needed. A number of HARQ processes are specified in the 3GPP TS 36.216 Specification already referenced above. These HARQ processes are laid out on the subframes available for backhaul transmissions, and used so that retransmissions are synchronous with respect to the HARQ process.
There are times at which the subframe configuration is changed by the donor base station node. When the subframe configuration is changed, the change may affect (e.g., also change) one or both of (1) the number of HARQ processes (e.g., the number of HARQ processes used on the uplink (UL) over the Un interface and (2) a mapping of HARQ processes to subframes.
Earlier releases of Long Term Evolution (LTE) do not contemplate a change in the number of HARQ process, and thus heretofore ramifications of such change have been unappreciated and not addressed. Thus, with the introduction of a specific subframe configuration and its potential change, it is not clear from prior practice what may happen to any data which may be pending in HARQ buffers of the HARQ processes. This ambiguity or uncertainty can lead to a mismatch in the HARQ process state between the donor base station node and the relay base station node, and to data loss.